The present invention relates to a printer including a plurality of print drums arranged to produce a color print by passing a paper sheet or similar recording medium only once, and a device for causing a plurality of rotary members to rotate in synchronism with each other.
Today, a stencil printer capable of producing a great number of prints at low cost is extensively used. The stencil printer includes a plurality of print drums arranged side by side in a direction in which a paper sheet or similar recording medium is conveyed. The print drums each are assigned to a particular color. While a paper sheet is passed only once, an image of the first color to an image of the last color are sequentially transferred from the print drums to the paper sheet one above the other, completing a color image. While such a single pass system is more efficient than a system of the type replacing a print drum color by color, it has problems ascribable to a short distance between the print drums.
Specifically, an ink image transferred from an upstream print drum assigned to, e.g., a first color reaches the nip of a downstream print drum assigned to, e.g., a second color in a wet state. As a result, the ink image is transferred to a master or perforated stencil wrapped around the downstream print drum and then to the next paper sheet.
More specifically, the transfer of the wet ink of the first color to the master wrapped around the downstream print drum does not matter for the first paper sheet. As for the second paper sheet, however, the ink of the first color is transferred from the above master to an image of the first color transferred from the upstream print drum to the paper sheet (so-called retransfer). Retransfer, i.e., the overlap of ink of the same color is not critical in the aspect of image quality if free from positional deviation. However, if the retransferred image is deviated from the original image, an offset ghost appears on the paper sheet. For a given amount of deviation, an offset ghost causes a thick line to appear blurred and causes a thin line to appear doubled, lowering image quality to a critical degree.
Retransfer stated above is not avoidable with a single pass type of color printer. An offset ghost is, however, ascribable to the positional deviation of transfer and can therefore be accurately reduced if the upstream and downstream print drums accurately rotate in synchronism with each other for thereby conveying a paper sheet with accuracy.
To reduce an offset ghost, it has been customary to connect the upstream and downstream print drums as to drive. Japanese Patent Laid-Open Publication No. 4-329175, for example, teaches a system that connects the shafts of the print drums by using a plurality of gears. Japanese Patent Laid-Open Publication No. 7-17121, for example, proposes a system that connects the print drums by using timing pulleys and a timing belt.
The gear scheme is capable of reducing the deviation of an offset ghost. This scheme, however, uses a plurality of precision gears and therefore increases the production cost. The timing belt scheme produces an offset ghost and, moreover, aggravates deviation thereof, as will be described specifically later with reference to the accompanying drawings.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-62737.
It is therefore an object of the present invention to provide a printer capable of reducing an offset ghost while using a timing belt.
It is another object of the present invention to provide a printer capable of reducing an offset ghost at low cost while using a timing belt.
In a synchronous driving device including a drive member and a driven member, drive pulleys each being mounted on one of the drive member and driven member, and a timing belt passed over the drive pulleys for causing the drive member and driven member to rotate in synchronism with each other, the drive pulleys each has a pitch circle diameter and a number of teeth related as:
d/z less than 1
where d denotes a pitch circle diameter and z denotes a number of teeth.
In the above configuration, the drive pulleys each may have a pitch of 3 mm or less.
Also, in accordance with the present invention, in a printer including a plurality of print drums spaced from each other in the direction of paper conveyance, toothed drive pulleys each being mounted on one of the print drums, and a timing belt passed over the toothed drive pulleys for allowing the print drums to rotate in synchronism with each other, the toothed drive pulleys each has a pitch circle diameter and a number of teeth related as:
d/z less than 1
where d denotes a pitch circle diameter and z denotes a number of teeth.
In the above configuration, the drive pulleys each may have a pitch of 3 mm or less.
Further, in accordance with the present invention, a device for driving a drive side and a driven side in synchronism includes a rotary member with a toothed drive pulley positioned at the drive side, a rotary member with a toothed drive pulley positioned at the driven side, and a timing belt passed over the toothed drive pulleys for causing the rotary members to rotate in synchronism with each other. The toothed drive pulleys each has a pitch circle diameter and a number of teeth related as:
d/z less than 1
where d denotes a pitch circle diameter and z denoted a number of teeth.
In the above construction, the drive pulleys each may have a pitch of 3 mm or less.